1. Field of the Invention
The present invention relates to machine-building, and more particularly, to methods and apparatus for manufacturing steel spring leaves.
The present invention can be used to advantage in the manufacture of springs intended for transport means.
The invention is also suitable for application in the manufacture of leaf-type articles which undergo severe fatigue stresses in service.
2. Description of the Prior Art
There is known in the art a method for manufacturing steel spring leaves, which consists in a succession of the following process operations: induction heating of each spring leaf, its bending, cooling and tempering.
Induction heating of each spring leaf to hardening temperature is effected in an oval-shaped inductor wherein the leaves are stacked. During heating, leaves are moved inside the inductor in stacks, after which each heated leaf is transferred to a die for bending. Next, each leaf is hardened by cooling in oil, then tempered separately.
Springs manufactured in accordance with the prior-art method feature adequate strength in ordinary service. However, the mass of a spring is high and, in addition, the manufacture of said springs calls for a substantial consumption of metal.
The realization of the aforesaid method makes its automation very complicated. Automation of the method requires relatively large floorspace and a numerous servicing personnel.
There is also known an apparatus for manufacturing steel springs which comprises arranged in succession entry guide rollers, bending rollers and exit guide rollers. The bending rollers are set in pairs in a manner to accommodate spring leaves therebetween. The known apparatus also incorporates an inductor with cooler wherein a spring leaf is hardened after bending.
The cooler in the known device is located directly back of the inductor so that bending is effected on a short length of the spring leaf close in value to the inductor width. Bending of spring leaves throughout their length in the known device requires welding to each spring leaf of an auxiliary leaf pre-cambered to a specified radius since, a straight leaf cannot pass through the apparatus due to specific design of the latter. Leaves are passed through the known apparatus as described above, whereupon, after bending and heat treatment, the pre-cambered leaf is cut off.
The aforesaid apparatus ensures the manufacture of spring leaves by the above-described method.
However, the known apparatus is insufficiently productive due to the necessity for welding an auxiliary leaf (see, e.g., "Premyshlennoye primeneniye tokov vysokoi chastoti v elektrotermii".
The principal object of the invention is to provide a method and apparatus for manufacturing steel spring leaves, which makes it possible to reduce the mass of a spring while retaining a high strength thereof.
Still another no less important object of the invention is to provide a method and apparatus for manufacturing steel spring leaves, which insures enhanced production efficiency in terms of output of springs.
A further object of the invention is to provide a method and apparatus for manufacturing steel spring leaves, which will reduce the requirements in floorspace.
These and other objects are accomplished by the provision of a method for manufacturing steel spring leaves, comprising the steps of induction heating to hardening temperature of each spring leaf, its bending, hardening by cooling, and tempering; in accordance with the invention each spring leaf is cooled in a manner to achieve tempering, whereupon the leaf is surface-hardened to a depth between about 0.1 th to about 0.3.sup.th part of spring leaf gauge at least on one of its sides by means of surface induction heating.
This enhances the fatigue strength of the spring leaf being treated and thus provides a possibility for reducing the mass of springs due to the following two factors, namely: creation of residual compressive stresses and formation of a surface layer possessing high strength due to fine-grained structure.
It is common knowledge that residual compressive stresses, when added up with working tensile stresses, reduce the magnitude of the latter and thus enhance the strength of spring leaves in cyclic loadings.
Surface hardening, accomplished after self-tempering, gives rise to a layer of martensite of a particularly fine-grained structure. The resultant martensite structure carrying no structurally-free ferrite makes it possible to obtain, in subsequent surface hardening, particularly fine-grained austenite whose place is filled after cooling by martensite with grains measuring 1 to 3 .mu.m. This type of structure features a high strength and plasticity.
It is expedient that the cooling of each spring leaf be effected by jets of a fluid medium followed by self-tempering to a surface temperature of each spring leaf from about 100.degree. C. and to about 500.degree. C. during a period of time necessary to attain a difference between the temperatures of the core of each spring leaf and its surfaces of about 30.degree. C. to about 100.degree. C.
Due to jet cooling with self-tempering, spring leaves may be quenched by water instead of oil as in the prior-art method. It has been experimentally prooved that in jet cooling combined with self-tempering, the use of water for quenching causes no cracks.
In addition, water quenching eliminates the need for flushing of leaves, which is imperative after oil quenching.
Moreover, self-tempering at the expense of heating that precedes surface-hardening lowers the consumption of electric power.
As the difference in the temperatures of the core and of surfaces of spring leaves prior to surface heating ranges from 30.degree. to 100.degree. C., the core of spring leaf is tempered during surface induction heating at sufficiently high temperatures required for provision of high residual compressive stresses in surface layers of spring leaves.
It is good practice to direct jets of a fluid at an angle to the surface of spring leaves of about 20.degree. to about 60.degree., and to introduce additional jets of a fluid upon the surface of spring leaves at angle of about 160.degree. to about 120.degree. into the stream of said medium on the surface of spring leaves formed by the initially supplied jets.
In case jets are directed only at an angle of between about 20.degree. and about 60.degree., there is formed a stream that spreads over the leaf, the latter undergoing in this case nonuniform cooling.
Provision of additional jets in a direction opposite to the initial ones at angle of 160.degree. to 120.degree. to the surface of leaves gives rise to a stream that blocks the stream formed by the initial jets.
The effect is a cooling zone that is stabilized, well-defined and adjustable through the spacing of the points of introduction of both streams.
The restruction of the cooling zone stabilizes self-tempering conditions and makes it possible to obtain in surface induction heating a hardened layer of a constant depth throughout the length of each spring leaf.
It is expedient that in an apparatus for accomplishing the proposed method, comprising successively arranged entry live rollers, bending rollers set in pairs for accommodating a spring sheet in-between, exit guide rollers, an inductor with cooler, in accordance with the invention, the inductor be arranged between the entry live guide rollers, and the cooler, between the bending and the exit guide rollers, and additionally be provided an inductor with cooler arranged-intermediate the exit guide rollers and intended for effecting surface-hardening of spring leaves; the cooler, the additional inductor with cooler and the exit guide rollers being arranged along a circular arc whose radius is equal to that of a specified radius of curvature of a spring leaf.
Such apparatus makes it possible to bend to a specified radius of curvature relatively short spring leaves without having to weld onto them pre-cambered spring leaves and to carry the proposed method into effect, which enhances the strength of spring leaves and thus reduces the mass of springs.
It is good practice to provide stops between the entry live guide rollers and the bending rollers and between the adjacent pairs of bending rollers at a distance from the surface of each leaf as it passes between the said rollers of about 0.0001-th to about 0.00005-th part of a specified radius of curvature of each spring leaf.
Said stops and their arrangement make it possible to bend to a specified radius of curvature the whole of a leaf, inclusive of its ends.
The use of the herein proposed method and apparatus for manufacturing steel spring leaves, makes it possible to reduce the mass of springs while retaining their adequate strength.
The invention enhances the productivity in spring leaf manufacture and facilitates the automation of all the constituent operations.
In addition, the method and apparatus of the invention bring down the requirements in floorspace and improve working conditions of attending personnel.